Back-off tool assembly



Jan. 27, .1970 w. aswEETMAN 3,491,830

BACK-OFF TooLAssEMBLY Filed April 5, 1968 i2.shams-sheet 1 l ATTORNEYJan. 27, `1970 wfG. s'wEETMAN Filed April 5, 1968 BACK-OFF TOOL ASSEMBLY2 SheetsfSheet 2 1MM/fam 6. Jwee fwd/7 INVENTQR.

BY A

ATTORNEY United States Patent O 3,491,830 BACK-OFF TOOL ASSEMBLY WilliamG. Sweetman, 5696 Longmont Drive, Houston, Tex. 77027 Filed Apr. 5,1968, Ser. No. 719,198 Int. Cl. E21b 23/04 U.S. Cl. 166-63 4 ClaimsABSTRACT OF THE DISCLOSURE An explosive jarring tool for releasingthreaded connections between sections of a pipe string in a well,employing detonating explosive encased in glass or ceramic casingadapted particularly for use under high temperature, high pressureconditions.

In rotary drilling of wells, drilling strings are conventionallyemployed which are made up of sections of pipe connected together byheavy screw couplings, commonly called tool joints. Such tool jointsconventionally comprise an internally threaded box member and anexternally threaded pin member adapted to be screwed tightly into thebox member when the joints are made up. Such drilling strings frequentlybecome stuck tightly in a well, whereupon it becomes highly desirable toretrieve maximum amounts of the pipe above the point at which the stringis stuck. Unscrewing of the string from the top of the well is generallyunsatisfactory because there is no way of assuring that the lowermostjoint above the point at which the pipe is stuck will be the one whichwill unscrew to thereby permit maximum recovery. Also because of the thetightness with which the joints are normally made up, or become set upduring use, the Unscrewing torque required will often lead totwisting-Gif of the string, which will result in excessive losses anddamage to the pipe.

The common method employed to effect release of the pipe is to locatethe stuck point and a collar above the stuck point at which the pipe isto be released. Such locating devices are well known and may be made apart of the tool string employed for releasing the connection. Releaseof the connection is effected by applying sufcient reverse torque to thepipe string at the top of the well to subject the connection toUnscrewing torque, and then applying a jarring blow to the connection bysettingoff an explosive force at or near the so-stressed connection.

These explosive releasing devices have heretofore been made in variousforms, a conventional form comprising a flexible tube, commonly calledPrimacord, lled with a suitable high explosive, set olf by an electricaldetonating cap. This system, while generally satisfactory when usedunder relatively low temperature, low pressure conditions, has provenunsatisfactory where high temperatures and high pressures are existentat the point at which the pipe is to be released. In many deep wells thetemperatures may attain 400 F. or more, and pressures of severalthousand pounds per square inch. In an effort to meet these conditions,some conventional back-off tools encase the explosive in metal casings,but these necessarily produce a substantial amount of junk when theexplosive is set off, which may seriously interfere with subsequentoperations in the well.

In other instances, the Primacord-type of explosive is used in which thecord is sheathed in soft lead tubing. While this type of protectionmaybe effective under relatively high temperatures, the lead sheathingwill be crushed under the high hydrostatic pressures, which normallywill be present, and the crushing force transmitted tothe explosive. Theresultant crushing of the explosive will cause the latter to burn orexplode low 3,491,830 Patented Jan. 27, 1970 ICC order rather thandetonate at high order, thereby rendering the shot ineffective for thepurpose of providing the necessary jarring force on the connection.

The present invention has for its primary object, the provision of aback-olf tool which employs a high temperature resistant detonatingexplosive encased in a tubular casing constructed of high pressure, hightemperature resistant glass or ceramic material which will disintegratesubstantially completely under the force of the explosive, therebyreducing or substantially eliminating any objectionable junk in thewell, while assuring high order detonation of the explosive such as toapply the necessary jarring force to the threaded connection to bereleased.

In accordance with this invention, high temperature detonatingexplosives, either in the form of cord or in the form of pellets, willbe inserted into a tubular casing constructed of an appropriate glass orceramic material adapted to withstand the high pressure, hightemperature conditions to be encountered. Because of the frangiblenature of the casing, the latter is provided with external protectorsmade of high temperature resistant, resilient material, such as siliconerubbers. These may be in the form of short sleeves mounted at spacedpoints along the casing, or as a continuous envelope covering the entireexterior of the casing and of suilicient thickness to serve as aresilient padding capable of preventing breakage of the casing bybumping of the casing against the wall of the well pipe, which may occuras it is being lowered through the well pipe.

Other and more specific objects and advantages of this invention willbecome more readily apparent from the following detailed descriptionwhen read in conjunction with the accompanying drawing which illustratespreferred embodimentsV in accordance with this invention.

In the drawing:

FIG. 1 is an elevational view of the back-off tool assembly shown insidea well pipe string adjacent a threaded coupling;

FIGS. 2A, 2B, and 2C, together constitute a longitudinal, sectional viewof the tool assembly;

FIG. 3 is a cross-sectional view taken along line 3 3 of FIG. 2C; and

FIG. 4 is a longitudinal, sectional View corresponding generally to theportion of the assembly illustrated by FIG. 2C, but showing a modicationof this portion of the assembly.

Referring to the drawing, there is shown a portion of a well W in whicha pipe string P is stuck at a point S and in which it is desired toback-off the pipe string at a threaded tool joint T. The tool assembly,designated generally by the numeral 10, is shown installed in the pipestring P with the explosive jarring element positioned opposite the tooljoint.

Referring now to FIGS. 2A, 2B and 2C, which, together, comprise theentire back-off tool assembly, the major parts, starting at the lowerend, include the explosive jarring element, designated generally by thenumeral 11, the upper end of which is connected to the firing head,designated generally by the numeral 12, which is, in turn, connected toa conventional rope socket, designated generally by the numeral 13,which is connected to the conventional lowering cable 14 which carriesan electrical conductor 15 by which current is supplied from the surfaceto the tiring head.

Firing head 12 includes a tubular body made up of a lower part 16 and anupper part 17 having a reduced diameter pin 18 at its lower end adaptedto be telescopically received in the bore 19 of lower part 16. Upper andlower parts 16 and 17 are releasably connected by means of the capscrews 20 which extend through openings 21 in the upper end of part 16and are screwed into pin 18. The upper end of upper part 17 is reducedin diameter to form an externally threaded pin 22 to be received in theinternally threaded box portion 23 of rope socket 13. An electricalcontact 24 is mounted in the upper end of pin 22 for engagement by anelectrical contact 25 carried in rope socket 13 and connected toconductor 15.

The lower end of firing head 12 is connected to jarring element 11 bymeans of a tubular coupling 30 having an axial bore 31, the lower end ofwhich is counterbored at 32 to receive the upper end of jarring tool 11.The upper end of coupling 30 is reduced in diameter to form a pin 33which is telescopically received in the bore of lower part 16 andreleasably secured thereto by means of set screws 34 which extendthrough the wall of lower part 16 into an annular recess 35 formed inthe exterior of coupling pin 33.

Jarring element 11 comprises a cylindrical tubular body 40, having abore 41, open at its upper end and closed at its lower end by anintegral hemispheric wall 42. Body 40 is constructed of glass or ceramicmaterial having highstrength properties adapted to resist both highexternal pressures and high temperatures but which when subjected to theshock of internally detonated explosive, will shatter completely intovirtually a powder. Such materials include Pyrex glass materials,particularly lCode 7740, and Pyroceram ceramic compositions,particularly Code 8605l (opaque) both manufactured by Corning Glass Co.,Corning, N.Y. Various products sold under these trademarks and similarknown products of other manufacturers, are capable of withstandingexternal hydrostatic pressure of as much as 25,000 lbs. per square inchand temperatures well above 400 F., conditions well above those likelyto be encountered in oil wells.

As previously noted, the upper end of tbody 40 is inserted intocounterbore 32 and is stronglysecured and sealed to collar 30 by meansof a high temperatureresistant epoxy-type sealing adhesive 44.

Bore 41 of body 40 has mounted therein one or more strands of a suitabledetonating cord 45, commonly called Primacord, which may be constructedof any conventional high temperature-resistant detonating explosiveenclosed within a suitable and well known sheathing material. Thesections of Primacord 45 will generally extend the full length of body40 and will be arranged in parallel and in longitudinal contact witheach other. The number of strands of Primacord will be a measure of thequantity of explosive employed and this will, in turn, be selected inaccordance with the hydrostatic pressure of the uid expected to beencountered in the Well through which the shock is to be imparted to thecoupling. The exterior of body 40 will have mounted thereon a pluralityof longitudinally spaced protector sleeves 46, the lowermost one 47being shaped to encase the lower end of the body defined by end Wall 42.These protectors will be constructed of any suitable resilient, hightemperature-resistant materials, such as the known silicone rubbersadapted to serve as eective shock absorbers. As many will be employed asmay be needed to assure protection against breakage of body 40 as aresult of contact with the metal Walls of a surrounding well pipethrough which the tool is run.

The mechanism employed for iiring the explosive Primacord is mounted inthe bore of the tiring head and includes a tubular bushing 50 having anaxial bore 51 which is screwed into the upper end of bore 31 of collar33 and an electrode 52 mounted in bore 51. The upper end of electrode 52is connected by an electrical connector 53 carried by one end of aninsulated conductor cable 54, the other end of which is connected tocontact 24. A conventional electrically iired explosive cap 49 has onelead 55 connected to electrode 52 and a second lead 56 connected as aground to the lower end of bushing 50. Cap 49 is placed in initiatingcontact with a pellet 48 of a suitable booster explosive which ispositioned in bore 41 in direct contact with the upper ends of theseveral strands of Primacord.

With this arrangement it will be seen that when electrical current isapplied by conventional means to cable 15, the current will betransmitted through the several electrical connections to cap 49 to firethe same, setting-off booster explosive 48 and the several strands ofPrimacord. The explosive material employed in the detonating cord may-be any of the known explosives of this type which are especiallyadapted to detonate at high order under high temperatures of the rangelikely to be encountered in very deep wells. Examples of such explosivesinclude: Tetranitro-l, Tetrazadibenzocyclo-Octatetrene, Picryl Sulfone,and various other commercial explosives sold under the trademarks Tacot(Du Pont), and Thermex III and Thermex VI (Amcel Corp.).

In operation, the back-off tool assembly heretofore described will beinstalled in a tool string, which may include a conventional stuck-pointindicator, a collar locator, or both. The tool string will be lowered on-cable 14 into the bore of the pipe string to the predetermined point atwhich the explosive is to be set off. Body 40 will normally be made twoto three feet in length in order to accommodate some margin of error indetermining the exact location of the threaded tool joint which is to bereleased. When the jarring element is suitably located adjacent theselected joint, current will be supplied from the surface byconventional means to set-off cap 49 and this, in turn, will explodebooster 48 and the main charge of explosive constituted by thePrimacord. Detonation of the latter will shatter body 40 to a powderymaterial and the shock resulting from the detonation of the Primacordwill be transmitted through any uid in the pipe string to the threadedjoint forming the coupling to be released. As indicated previously, thisjoint will be under the stress of unscrewing torque applied at thesurface to the section of pipe P above selected tool joint T, and theshock of the explosion will result in release of the threads, permitingthe portion of the pipe string above tool joint T to be unscrewed andremoved from the well.

By making jarring element 11 of highly friable material, such as theglass or ceramic materials listed, only that element will be destroyedin the the course of operation of the tool assembly and all of the metalstructure represented by tiring head 12 and the other parts connectedthereabove can be recovered substantially intact and in conditionenabling their re-use upon attachment of a new jarring element andexplosive cap.

FIG. 4 illustrates a modification of jarring element and is designatedgenerally by the numeral 111. In this modiication, the glass or ceramictube is identical with that previously described. The bore of tube 140is illed with a stack of pellets 148 constructed of hightemperature-resistant explosive similar to that used in the Primacord,but constructed in the form of cylindrical pellets illustrated. Thepellets are preferably encased in a loading cylinder constructed ofcardboard or similar light destructible material, serving primarily as asupporting container for the pellets.

In this modification, the explosive cap will be directed against theuppermost pellet or `an intervening body of a suitable booster explosive(not shown), as will be Well understood by those skilled in the art.

In this modification, tube 140l is illustrated as sheathed throughoutits length with a continuous protective, shockabsorbing sheath 146constructed of high temperature-resistant silicone rubber, as previouslydescribed. It will be understood that the spaced sleeve protectors, suchas previously described, may be employed instead yof the continuoussheath.

It will be understood that various other alterations and modiiicationsmay be made in the details of the illustrative embodiments within thescope of the appended claims but without departing from the spirit ofthis invention.

What I claim and desire to secure by Letters Patent is:

1. Apparatus for releasing a threaded joint between sections of a wellpipe string under well conditions of highpressure and high-temperature,comprising:

(a) an elongate tubular body adapted to be positioned longitudinallywithin a threaded joint;

said body being constructed of a material selected from the groupconsisting of Pyrex glass and Pyroceram ceramic materials characterizedby high-pressure, high-temperature-resistant properties but highlyfriable when subjected to an internally applied explosive force;

(b) an elongate charge of high temperature-resistant detonatingexplosive enclosed within and extending longitudinally substantially thefull length of said body and operable when exploded to shatter said bodyand apply an effective thread-releasing jarring force to said joint; and

(c) explosive initiating means mounted on the body in detonationrelation to said charge;

said explosive being in the form of a plurality of parallel lengths ofPrimacord extending substantially the full length of said body anddisposed in continuous longitudinal Contact with Aeach other..

2. Apparatus according to claim 1 wherein said explosive initiatingmeans is of the electrically red type.

3. Apparatus according to claim 1 having resilent shock absorber meansmounted about the exterior of said body.

4. Apparatus according to claim 1 having a body of a booster explosivedisposed in boosting contact between said initiating means and theadjacent end of said charge.

References Cited UNITED STATES PATENTS 4/ 1967 Mohaupt 166-63 X DAVID H.BROWN, Primary Examiner.

U,S. Cl. X.R, 166-301

